Fibers (active fibers) to which laser active materials are added are used in laser oscillators and light amplifiers. For example, in laser oscillators and light amplifiers in which active fibers made of fluoride are used, in order to cause laser oscillation to occur, pump light is introduced into the fibers from an end surface of the fibers. When pump light is guided in an active fiber, the pump light is absorbed by a core portion to which a laser active material is added, and thus, output light is emitted. At this time, absorption of the pump light by the core causes heat generation in the core.
In particular, compared with commonly used quartz fibers, fluoride fibers have poor thermal resistance, and therefore, in the case of intense pump light, the fibers are deformed or damaged by the generated heat, and it becomes unable to cause laser oscillation and amplification to occur. For this reason, with a configuration in which pump light is introduced into a fiber from an end surface of the fiber, laser output is limited.
To address this issue, it is conceivable to use an optical coupler. A device may have a gain fiber and a pump light fiber connected to a side surface of the gain fiber. The pump light fiber guides pump light from a pumping source to the gain fiber. If a laser oscillator is configured by connecting a plurality of units in each of which a pump light fiber is connected to a side surface of a gain fiber as described above, pump light can be introduced from a plurality of portions. Thus, it is possible to increase the power of pump light that can be introduced, and hence increase the laser output while suppressing heat generation in the gain fiber.